gengo: add unmarshall test in generated package.

Exercise unmarshal against our generated "Strang" type.  Works!

(Note: this will fail if you *haven't* run the codegen yet; but it's in
an underscore-prefixed package, so it won't run unless you dive in here
intentionally.  Overall, this continues to be early-days hijinx, and
does not represent a good long-term testing strategy.)

schema/gen/go/_test/wow_test.go

+package whee
+
+import (
+	"testing"
+
+	"github.com/polydawn/refmt/tok"
+	. "github.com/warpfork/go-wish"
+
+	ipld "github.com/ipld/go-ipld-prime"
+	"github.com/ipld/go-ipld-prime/encoding"
+	"github.com/ipld/go-ipld-prime/fluent"
+)
+
+// TokenSourceBucket acts like a TokenSource by yielding tokens from a pre-made
+// slice; and also keeps track of how far it's been read.
+type TokenSourceBucket struct {
+	tokens []tok.Token
+	read   int
+}
+
+func (tb *TokenSourceBucket) Step(yield *tok.Token) (done bool, err error) {
+	*yield = tb.tokens[tb.read]
+	tb.read++
+	return tb.read > len(tb.tokens), nil
+}
+
+func TestScalarUnmarshal(t *testing.T) {
+	t.Run("string node", func(t *testing.T) {
+		tb := &TokenSourceBucket{tokens: []tok.Token{
+			{Type: tok.TString, Str: "zooooom"},
+		}}
+		nb := Strang__NodeBuilder{}
+		n, err := encoding.Unmarshal(nb, tb)
+		Wish(t, err, ShouldEqual, nil)
+		Wish(t, n.ReprKind(), ShouldEqual, ipld.ReprKind_String)
+		Wish(t, fluent.WrapNode(n).AsString(), ShouldEqual, "zooooom")
+		Wish(t, tb.read, ShouldEqual, 1)
+	})
+}